rss, iflib: document that RX queue counts are not power-of-two

[nick_spun.io]

Summary

Drivers that build an RSS indirection table sometimes assume the RX queue count is a power of two and reduce buckets to queues with an AND mask, which starves queues when the count is not one (iflib does not guarantee a power-of-two queue count). Record the constraint at the two places a driver author is likely to look:

• rss_get_indirection_to_bucket() in the RSS framework
• the ifdi_rx_queues_alloc() KPI in iflibdd(9)

Documentation only; no functional change.